After upgrading mpy from 1.17 to 1.18, my buzzer can't play low frequency sounds any more.

With further test, it shows that the pwm has no output when frequencies < 611Hz.

from machine import PWM, Pin
pwm = PWM(Pin(5)) # also tested with Pin(2)
pwm.duty(512)
pwm.freq(611) # it works, digital multimeters shows freq 1222Hz
pwm.freq(610) # no pwm output, digital multimeters says freq is 0

And after switching back to mpy 1.17, pwm.freq(610) works fine. I'v tested with another board and I still got the same result. I'm sure this is an issue introduced by v1.18.